a certain geometric layout of the radio frequency coils and sophisticated electronics.
Advanced computer facilities are needed for operation of NMR instruments, as well as
analysis of the acquired spectra.13.5.3 Applications
Molecular structure determination
Traditionally, NMR spectroscopy is the main method of structure determination for
organic compounds. The chemical shift provides a clue about the environment of a
particular proton or carbon, and thus allows conclusions as to the nature of functional
groups. Spin–spin interactions allow conclusions as to how protons are linked with
the carbon skeleton. For structure determination, the fine structure usually is the most
useful information because it provides a unique criterion while chemical shifts of
some groups can vary over an extended range. Additionally, the signal intensity
provides information as to how many protons contribute to a particular signal.Solution structure of proteins and peptides
The structures of proteins up to a mass of about 50 kDa can be determined with
biomolecular NMR spectroscopy. The development of magnets with very high field
strengths (currently 900 MHz) continues to push the size limit. The preparation of
proteins or selected domains for NMR requires recombinant expression and isotopic
labelling to enrich the samples with^13 C and^15 N;^2 H labelling might be required as
well. Sample amounts in the order of 10 mg used to be required for NMR experiments;
however, the introduction of cryoprobe technology has reduced the sample amount
significantly. Heteronuclear multidimensional NMR spectra need to be recorded for
the assignment of all chemical shifts (^1 H,^13 C,^15 N). For interproton NOEs,^13 C- and(^15) N-edited 3D NOESY spectra are required. The data acquisition can take several
weeks, after which spectra are processed (Fourier transformation) and improved with
respect to digital resolution and signal-to-noise. Assignment of chemical shifts and
interatomic distances is carried out with the help of software programs. All experi-
mentally derived parameters are then used as restraints in a molecular dynamics or
simulated annealing structure calculation. The result of a protein NMR structure is an
ensemble of structures, all of which are consistent with the experimentally determined
restraints, but converge to the same fold.
Magnetic resonance imaging
The basic principles of NMR can be applied to imaging of live samples. Because the
proton is one of the more sensitive nuclides and is present in all biological systems
abundantly,^1 H resonance is used almost exclusively in the clinical environment.
The most important compound in biological samples in this context is water. It is
distributed differently in different tissues, but constitutes about 55% of body mass in
the average human. In soft tissues, the water distribution varies between 60% and
90%. In NMR, the resonance frequency of a particular nuclide is proportional to the
543 13.5 Nuclear magnetic resonance